How to Check the Battery Health of a Used Electric Car
The battery is the single most valuable part of any used electric car, and it is also the one part you cannot see. On a petrol car you can hear the engine, check the service history and kick the tyres. On an EV the thing that decides whether the car is worth the money is sealed inside the floor, and it has been quietly ageing since the day it was built.
So how do you actually check it before you hand over your money? This guide walks through the options, from the free ones that tell you very little to the independent test that gives you proof.
Why the Dashboard Range Tells You Almost Nothing
The first instinct most buyers have is to look at the range figure on the dashboard. It feels like the obvious place to start. It is also close to useless as a measure of battery health.
That range number, often nicknamed the "guess-o-meter", is a short-term prediction. It is based on how the car has recently been driven, the outside temperature, the climate settings and a dozen other moving parts. Take the same car on a cold motorway run with the heater on and the number drops. Drive it gently around town in mild weather and it climbs. None of that reflects the true, long-term condition of the battery.
A car that shows a healthy-looking range on a warm forecourt afternoon can still have a tired battery underneath. The range display was never designed to be a health report, and you should not treat it as one.
The Car Marks Its Own Homework
The next step buyers take is to look at the car's own battery health readout, if the model shows one. This is better than the range figure, but it comes with a catch that is easy to miss.
That percentage is calculated by the car's own battery management system, using the manufacturer's own software and its own assumptions. In other words, the car is grading itself. There is no independent check on the method, no agreed standard behind the number, and no way for you to verify how it was reached.
Manufacturers also differ in what they even mean by "health". Some report usable capacity, some report a raw figure, and the way the system estimates it can drift over time. Two cars showing the same percentage can be in genuinely different condition. We go into why these figures vary so much in State of Health Explained.
None of this makes the car's readout dishonest. It simply means it is not proof, and it is not something you would want to rely on when thousands of pounds are on the line.
What an Independent Test Actually Measures
This is the gap that independent battery testing fills. Instead of asking the car what it thinks of itself, an independent test reads the raw data out of the battery and assesses it against a consistent standard.
The key difference is depth. A good independent test does not just produce one overall number. It looks at the pack cell by cell, or module by module, and checks how evenly the battery is performing. That matters, because a battery is only as strong as its weakest cells. A pack can show a reasonable overall figure while hiding a group of weak or unbalanced cells that will drag it down over time. A cell-level view catches that; a single dashboard percentage never will.
The test we point buyers towards is the AVILOO FLASH test. It is a 3-minute check that plugs into the car's OBD-2 port, and the result is a certificate carrying a TÜV Certified State of Health reading. Because it is carried out by an approved tester and assessed independently, the number on the certificate is not the car marking its own homework. It is an outside verdict. Independent testing is worth doing precisely because it removes the conflict of interest, which is the whole argument we make in why an independent test matters.
Your Options as a Buyer
In practice you have a few realistic ways to check battery health before buying.
Ask the seller for a certificate
More dealers are now having their used EVs tested and can show you a battery health certificate on request. If the car already has one, read it properly. Check that it is recent, that it names the vehicle, and that it comes from an independent test rather than a screenshot of the car's own display. You can see what a genuine certificate looks like and what each part of it means on the certificate page.
Book your own independent test
If there is no certificate, or you simply want your own peace of mind, you can arrange an independent test yourself. The certified test costs £100, paid to the approved tester on the day, with nothing to pay to book. For a car costing many thousands of pounds, having proof of the condition of its most expensive component is a small price to pay.
Walk away from a flat refusal
A seller who will not have the car tested, and cannot explain why, is telling you something. Plenty of dealers are happy to arrange or allow a test. A firm no, on the one part of the car you cannot inspect yourself, is worth taking seriously.
What the Test Involves and What You See
The FLASH test itself is quick and undramatic. The tester plugs a device into the OBD-2 port, usually found under the dashboard, and reads the battery data while the car sits there. Three minutes later the certificate is produced.
The certificate shows the State of Health as a percentage, backed by the cell-level analysis behind it, along with the vehicle details so the result is tied to that specific car. Because it is TÜV Certified and issued independently, it is something you can keep, show to an insurer, or pass on if you ever sell the car.
What a Healthy Result Looks Like
At a high level, most used EVs lose a chunk of capacity in the first year and then settle into a slower, gentler decline. What you are really watching for is a figure that is lower than the car's age and mileage would explain, or a pack where the cells are performing unevenly rather than together.
We have written a separate guide on what counts as a good percentage and how to read the number in context, so we will not repeat the detail here. The short version is that the headline figure only means something once you know it came from an independent, cell-level test rather than the car's own estimate.
Checking the battery health of a used EV comes down to one idea: trust proof, not the car's own opinion of itself. The range display and the built-in readout are starting points, not evidence. An independent, cell-level test is what turns a hopeful guess into something you can actually rely on.
Ready to know for certain? You can book a £100 certified battery test and have an independent verdict on the car before you buy.